1. Field of the Invention
The embodiments of the invention relate to a liquid crystal display device, and more particularly, to a liquid crystal display device including a backlight unit and a method of driving the liquid crystal display device.
2. Discussion of the Related Art
A liquid crystal display (LCD) device includes a liquid crystal display panel and a backlight unit. The liquid crystal display panel includes a plurality of liquid crystal cells disposed in matrix and a plurality of thin film transistors (TFTs) through which image signals are supplied. Rotation angles of liquid crystal molecules in each liquid crystal cell as well as transmittance through each liquid crystal cell are controlled according to the image signals so as to display images on the liquid crystal display panel.
A cold cathode fluorescent lamp (CCFL) is used as a light source for the backlight unit of an LCD device. The backlight unit should be designed to have a thin profile and light weight while providing a large amount of light to the liquid crystal display panel. Accordingly, a light emitting diode (LED) has been suggested for use in a backlight due to an LED having the characteristics of low power consumption, light weight and high brightness compared to a CCFL.
FIG. 1 is a view showing an edge type backlight unit according to the related art. As shown in FIG. 1, a backlight unit includes a plurality of LED arrays 10 each having a plurality of LEDs 12 and an LED driving unit 20. A pulse width modulation (PWM) signal is supplied to the LED driving unit 20 from an external circuit unit (not shown). The plurality of LED arrays 10 are turned ON/OFF according to a power supplied synchronous with an ON time period of the PWM signal while the liquid crystal display panel displays images. The plurality of LED arrays 10 driven by the PWM signal (PWM driving) has advantages in power consumption and in color rendering over a plurality of LED arrays that are always turned ON by driving with a constant direct current (DC) voltage, otherwise known as DC driving.
Since the single PWM signal is supplied to the LED driving unit 20 and the plurality of LED arrays 10 are controlled by the single PWM signal, the plurality of LED arrays are simultaneously turned ON/OFF. Each TFT in the liquid crystal display panel is formed of amorphous silicon. When light enters the amorphous silicon, a photo leakage current corresponding to the intensity of the light from the LED arrays 10 is generated in the amorphous silicon so as to affect the OFF current in each TFT. Accordingly, when the plurality of LED arrays 10 is switched ON and OFF by the PWM signal, each TFT of the liquid crystal display panel will have variations in their OFF current. For example, the OFF current of each TFT when the plurality of LED arrays 10 are turned ON will vary to be greater than the OFF current of each TFT when the plurality of LED arrays 10 are turned OFF. The variations in the OFF current in the TFTs cause deterioration in the image display quality of the liquid crystal display panel. For example, the deterioration can be waviness or noise in portions of the liquid crystal display panel displaying darker images and/or in other portions of the liquid crystal display panel displaying brighter images.